III-nitride materials including low dislocation densities and methods associated with the same

ABSTRACT

Semiconductor structures including one, or more, III-nitride material regions (e.g., gallium nitride material region) and methods associated with such structures are provided. The III-nitride material region(s) advantageously have a low dislocation density and, in particular, a low screw dislocation density. In some embodiments, the presence of screw dislocations in the III-nitride material region(s) may be essentially eliminated. The presence of a strain-absorbing layer underlying the III-nitride material region(s) and/or processing conditions can contribute to achieving the low screw dislocation densities. In some embodiments, the III-nitride material region(s) having low dislocation densities include a gallium nitride material region which functions as the active region of the device. The low screw dislocation densities of the active device region (e.g., gallium nitride material region) can lead to improved properties (e.g., electrical and optical) by increasing electron transport, limiting non-radiative recombination, and increasing compositional/growth uniformity, amongst other effects.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/886,506, filed Jul. 7, 2004, which is incorporated herein byreference in its entirety.

FIELD OF INVENTION

The invention relates generally to III-nitride materials and, moreparticularly, to III-nitride (e.g., gallium nitride) material structuresincluding low dislocation densities, as well as methods associated withthe same.

BACKGROUND OF INVENTION

III-nitride materials include gallium nitride (GaN), aluminum nitride(AlN), indium nitride (InN) and their respective alloys (e.g., AlGaN,InGaN, AlInGaN and AlInN). In particular, gallium nitride materials (GaNand its alloys) have attractive properties including the ability toefficiently emit blue light, the ability to transmit signals at highfrequency, and others. Accordingly, gallium nitride materials are beingwidely investigated in many microelectronic applications such astransistors, field emitters, and optoelectronic devices. Semiconductorstructures that include gallium nitride material regions oftentimesinclude regions of other III-nitride materials (e.g., AlN). Such layers,for example, may function as buffer or intermediate layers positionedbetween a substrate and an overlying gallium nitride material region.

III-nitride materials have different crystal structures and propertiesthan many common substrates including silicon, silicon carbide andsapphire. Thus, when III-nitride materials are formed on suchsubstrates, these differences may lead to the formation of defectsincluding dislocations. For example, dislocations may result fromdifferences between the lattice constants of the substrate and theIII-nitride material; differences between the thermal expansioncoefficients of the substrate and the III-nitride material; as well as,substrate quality including misorientation and defects.

Dislocations are linear imperfections in a crystalline array of atoms.Types of dislocations include edge dislocations, screw dislocations andmixed dislocations (which have an edge component and a screw component).The presence of dislocations in III-nitride materials that are in thevicinity of the active region in a device can impair device performance.For example, the dislocations can function as scattering centers whicheffect electron transport and, thus, impair electrical performance.Also, the dislocations can function as non-radiative recombinationcenters which reduce performance of opto-electronic devices.Furthermore, dislocations can lead to inhomogeneities in composition andformation of macro-defects which can also negatively effect deviceperformance.

Certain conventional vertical growth processes (i.e., growth thatproceeds in a vertical direction from the underlying layer) formIII-nitride material regions having screw dislocation densities ofgreater than about 10¹²/cm³. Lateral growth processes can producelocalized areas within III-nitride material regions having low defectdensities, while other areas within the regions have relatively highdefect densities. Lateral growth processes are typically more complexthan vertical growth processes.

SUMMARY OF INVENTION

The invention provides III-nitride materials having low dislocation(e.g., screw dislocation) densities including structures, devices andmethods associated with the materials.

In one embodiment, a semiconductor structure is provided. The structurecomprises a III-nitride material region having a screw dislocationdensity of less than about 10⁸/cm² throughout the III-nitride materialregion.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a III-nitride material region including an areahaving dimensions of at least about 100 microns×100 microns and a screwdislocation density of less than about 10⁴/cm².

In another embodiment, a semiconductor structure is provided. Thestructure comprises a semiconductor region having a top surface; and, aIII-nitride material region formed over the top surface and having adifferent composition than the semiconductor region. A cross-sectionalarea of the III-nitride material region within 100 nanometers of the topsurface has a screw dislocation density of less than about 10⁸/cm².

In another embodiment, a semiconductor structure is provided. Thestructure comprises a III-nitride material region having a screwdislocation density of less than about 10⁸/cm² and an edge dislocationdensity of greater than about 10⁸/cm².

In another embodiment, a semiconductor structure is provided. Thestructure comprises a III-nitride material region having an edgedislocation density and a screw dislocation density. The edgedislocation density is at least 100 times greater than the screwdislocation density.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a substrate, and a silicon nitride-based materiallayer having a thickness of less than 100 Angstroms and substantiallycovering an entire top surface of the substrate.

The structure further comprises a III-nitride material region formedover the silicon nitride-based material layer and having a screwdislocation density of less than about 10⁸/cm².

In another embodiment, a method of forming a semiconductor structure isprovided. The method comprises providing a substrate; and, forming agallium nitride material region over the substrate having a screwdislocation density of less than about 10⁸/cm² throughout theIII-nitride material region.

In another embodiment, a method of forming a semiconductor structure isprovided. The method comprises providing a semiconductor material regionhaving a top surface; and, vertically growing a III-nitride materialregion over the semiconductor material region. The III-nitride materialregion has a composition different than the semiconductor materialregion. A screw dislocation density in a cross-sectional area of theIII-nitride material region located within 100 nanometers of the topsurface of the semiconductor material region is less than about 10⁸/cm².

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. Theaccompanying figures are schematic and are not intended to be drawn toscale. In the figures, each identical, or substantially similarcomponent that is illustrated in various figures is represented by asingle numeral or notation. For purposes of clarity, not every componentis labeled in every figure. Nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow thoseof ordinary skill in the art to understand the invention. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a structure including III-nitride material regionshaving low screw dislocation densities according to one embodiment ofthe present invention.

FIG. 2 illustrates a structure including III-nitride material regionshaving low screw dislocation densities according to another embodimentof the present invention.

FIG. 3 illustrates a structure including III-nitride material regionshaving low screw dislocation densities according to another embodimentof the present invention.

FIG. 4 illustrates a free-standing III-nitride material region having alow screw dislocation density according to another embodiment of thepresent invention.

FIG. 5 illustrates a FET device including III-nitride material regionshaving low screw dislocation densities according to one embodiment ofthe invention.

FIG. 6 is a copy of a STEM (scanning transmission electron microscope)image that illustrates the presence of a silicon nitridestrain-absorbing layer between an aluminum nitride layer and a siliconsubstrate as described in Example 1.

FIGS. 7A and 7B are respective copies of bright field cross-sectionalTEM images of a structure as described in Example 1.

FIGS. 8A and 8B are respective copies of the TEM images of FIGS. 7A and7B with dislocations highlighted by dashed lines as described in Example1.

DETAILED DESCRIPTION

The invention provides semiconductor structures including one or moreIII-nitride material region(s) (e.g., a gallium nitride material region)and methods associated with such structures. The III-nitride materialregion(s) advantageously have a low dislocation density and, inparticular, a low screw dislocation density. In some embodiments, thepresence of screw dislocations in the III-nitride material region(s) maybe essentially eliminated. As described further below, the presence of astrain-absorbing layer underlying the III-nitride material region(s),amongst other factors, can contribute to achieving the low screwdislocation densities. In some embodiments, the III-nitride materialregion(s) having low dislocation densities include a gallium nitridematerial region which functions as the active region of the device. Thelow screw dislocation densities of the active device region (e.g.,gallium nitride material region) can lead to improved properties (e.g.,electrical and optical) by decreasing electron scattering, limitingnon-radiative recombination, increasing compositional uniformity, andreducing macro-defect formation, amongst other effects.

FIG. 1 illustrates a semiconductor structure 10 according to oneembodiment of the invention. The semiconductor structure includes astrain-absorbing layer 12 formed on a substrate 14. A series ofIII-nitride material regions are formed on the strain-absorbing layer.In this illustrative embodiment, the series of III-nitride materialregions includes an intermediate layer 15, a transition layer 16 and agallium nitride material region 18. As described further below, thestrain-absorbing layer (amongst other factors) may help contribute tolimiting generation of screw dislocation densities in at least one (and,in some cases, all) of the overlying III-nitride layers and, inparticular, in the gallium nitride material region. Semiconductorstructures of the invention may form the basis of a number ofsemiconductor devices including transistors (e.g., FET), Schottkydiodes, light emitting diodes and laser diodes, amongst others.

When a layer is referred to as being “on” or “over” another layer orsubstrate, it can be directly on the layer or substrate, or anintervening layer also may be present. A layer that is “directly on”another layer or substrate means that no intervening layer is present.It should also be understood that when a layer is referred to as being“on” or “over” another layer or substrate, it may cover the entire layeror substrate, or a portion of the layer or substrate.

As used herein, the term “III-nitride material” refers to any Group IIIelement-nitride compound including boron nitride (BN), aluminum nitride(AlN), gallium nitride (GaN), indium nitride (InN) and thalium nitride(TlN), as well as any alloys including Group III elements and Group Velements (e.g., Al_(x)Ga_((1-x))N, Al_(x)In_(y)Ga_((1-x-y))N,In_(y)Ga_((1-y))N Al_(y)In_((1-y))N, GaAs_(a)P_(b) N_((1-a-b)),Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b) N_((1-a-b)) and the like).Typically, when present, arsenic and/or phosphorous are at lowconcentrations (i.e., less than 5 weight percent). III-nitride materialsmay be doped n-type or p-type, or may be intrinsic.

As used herein, the phrase “gallium nitride material” refers to galliumnitride (GaN) and any of its alloys, such as aluminum gallium nitride(Al_(x)Ga_((1-x))N), indium gallium nitride (In_(y)Ga_((1-y))N),aluminum indium gallium nitride (Al_(x)In_(y)Ga_((1-x-y))N), galliumarsenide phosporide nitride (GaAs_(a)P_(b)N_((1-a-b))), aluminum indiumgallium arsenide phosporide nitride(Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), amongst others.Typically, when present, arsenic and/or phosphorous are at lowconcentrations (i.e., less than 5 weight percent). In certain preferredembodiments, the gallium nitride material has a high concentration ofgallium and includes little or no amounts of aluminum and/or indium. Inhigh gallium concentration embodiments, the sum of (x+y) may be lessthan 0.4, less than 0.2, less than 0.1, or even less. In some cases, itis preferable for the gallium nitride material layer to have acomposition of GaN (i.e., x+y=0). Gallium nitride materials may be dopedn-type or p-type, or may be intrinsic. Suitable gallium nitridematerials have been described in commonly-owned U.S. Pat. No. 6,649,287which is incorporated herein by reference.

As used herein, the term “region” may refer to one layer (e.g., layer15, layer 16) or may refer to a series of layers (e.g., gallium nitridematerial region 18 including layers 18 a, 18 b, 18 c).

At least one of the III-nitride material regions (e.g., intermediatelayer 15, transition layer 16, gallium nitride material region 18)formed on the strain-absorbing layer has low screw dislocationdensities. In some embodiments, it may be preferred for all of theIII-nitride material regions formed on the strain-absorbing layer tohave low screw dislocation densities (including the screw dislocationdensity values noted below). It may be particularly preferable for thegallium nitride material region to have a low screw dislocation densitybecause in many device applications the active device region is formedin the gallium nitride material region, as described further below.

However, it should be understood, that in certain embodiments, it may bepossible for one or more of these III-nitride material regions to havehigher screw dislocation densities.

In some embodiments, the screw dislocation density in one (or more) ofthe III-nitride material regions above the strain-absorbing layer isless than about 10⁸ screw dislocations/cm², and, in some cases, lessthan about 10⁶/cm². It is even possible to achieve significantly lowerscrew dislocation densities within the scope of the present invention.In some embodiments, one (or more) of the III-nitride material regionshave very low screw dislocation densities of less than about 10⁴/cm²and, in some cases, less than about 10²/cm². In some embodiments, it iseven possible for one (or more) of the III-nitride material regions tohave substantially zero screw dislocations.

As used herein, units for screw dislocation density may be expressed as“screw dislocations/cm²” or “/cm²”.

In some preferred embodiments, at least the gallium nitride materialregion has the low screw dislocation density values noted above.

The specific screw dislocation density depends, in part, on theparticular structure including factors such as the thickness,composition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the overlyingIII-nitride material layer(s)/region(s); as well as, the composition,thickness, and crystal structure of the substrate, amongst otherfactors.

Screw dislocation densities may be measured using known techniques. Forexample, a diffraction contrast technique (e.g., g-vector analysis),which involves comparing transmission electron microscope (TEM) imagesof the material structure obtained under different imaging conditions,may be used to selectively view the screw dislocations and measure thescrew dislocation density.

It should be understood that the screw dislocation densities describedherein are measured over a representative area. The representative areamay depend, in part, on the actual dislocation density being measured.For example, a representative area of about 100 microns squared (e.g.,10 micron×10 micron) may be suitable for measuring screw dislocationdensities on the order of 10⁸/cm². Larger representative areas may beused for smaller screw dislocation densities.

Advantageously, in some embodiments, the above-identified screwdislocation density ranges are present throughout one (or more) of therespective III-nitride material region(s) overlying the stress-absorbinglayer. That is, throughout the entire volume of at least one (or more)of the III-nitride material region(s), the screw dislocation densitiesranges identified above may be achieved.

It should be understood that, in some embodiments, all of theIII-nitride material regions (e.g., intermediate layer 15, a transitionlayer 16 and a gallium nitride material region 18) formed over thestrain-absorbing layer have the above-identified screw dislocationdensities.

The low screw dislocation densities are also achievable across largeareas of III-nitride material regions. For example, the above-identifiedscrew dislocation densities may be found in regions having an area ofgreater than about 10⁴ square microns; or, greater than about 1 mm². Insome cases, the areas of the III-nitride material regions having the lowscrew dislocation densities may be greater than about 1 cm², or more.

The above-noted areas may comprises a variety of dimensions (i.e.,lengths, widths) including similar lengths and widths. For example, theabove-identified screw dislocation densities may be found in areashaving dimensions of greater than about 100 microns×100 microns; or,greater than about 1 mm×1 mm. In some cases, the areas of theIII-nitride material regions having the low screw dislocation densitiesmay have dimensions of greater than about 1 cm×1 cm, or more.

The consistently low screw dislocation densities across large areasand/or throughout the entire region(s) provides advantages over certainprior art techniques (e.g., certain lateral growth processes) that form,within a respective III-nitride material region, areas of low screwdislocation density and also areas of high screw dislocation density(e.g., greater than 10⁸/cm²).

In some embodiments, the III-nitride material region may have asubstantially constant screw dislocation density throughout the entireregion and/or area. In some embodiments, the screw dislocation densityis substantially constant along an axial direction (i.e., z-axis inFIG. 1) and is also substantially constant along a radial direction(i.e., x-axis in FIG. 1).

The above-identified screw dislocation densities are achievable withinareas of the III-nitride material region proximate to (e.g., within 100nanometers, or within 50 nanometers) an underlying semiconductor region(e.g., substrate or semiconductor layer) formed of a different materialcomposition. The area, for example, is a cross-sectional area (e.g., A,in FIG. 1) of the III-nitride material region within 100 nanometers (or,within 50 nanometers) of the upper surface of the underlying region. Thecross-sectional area may extend across the III-nitride region and may beparallel to the lower surface of the III-nitride material region (or theupper surface of the underlying region). The cross-sectional area mayhave any of the area values described above. The cross-sectional areaand the III-nitride material region may be directly above the underlyingsemiconductor region, as shown.

In these embodiments, strain-absorbing layer 15 (with a thickness ofless than 100 nanometers) may be formed between the III-nitride materialregion and the underlying semiconductor region (e.g., substrate 14).

In some cases, above-identified screw dislocation densities areachievable within an area of the III-nitride material region formedwithin 100 nanometers (or, 50 nanometers) of the top surface of asubstrate having a different material composition than the III-nitridematerial region. In these cases, the substrate may be formed of silicon,silicon carbide, sapphire (or, other substrate materials described belowthat meet this condition). The III-nitride material region may beintermediate layer 15, transition layer 16 or gallium nitride materialregion 18, depending on the structure.

In some cases, above-identified screw dislocation densities areachievable in a III-nitride material region formed within 100 nanometers(or, 50 nanometers) of the top surface of a semiconductor layer (i.e.,non-substrate) having a different material composition than theIII-nitride material region. For example, the semiconductor materiallayer may be formed of a III-nitride material having a differentcomposition than the III-nitride material region.

The mechanism for limiting/eliminating screw dislocations in structuresof the present invention may not have as significant an effect onlowering the edge dislocation density or mixed dislocation density. Forexample, in some embodiments, the edge dislocation density and/or mixeddislocation density may be greater than about 10⁸/cm² in the III-nitridematerial region having the reduced screw dislocation density. In somecases, the edge dislocation density and/or mixed dislocation density isat least 100 times, or at least 10⁴ times, greater than the screwdislocation density in the III-nitride material region. In embodimentshaving very low screw dislocation densities, the edge dislocationdensity and/or mixed dislocation density may be at least 10⁶, or atleast 10⁸ times, the screw dislocation density in the III-nitridematerial region. In embodiments that do not have low edge and/or mixeddislocation densities, it should be understood that the structures stillhave the above-identified advantages associated with lower screwdislocation densities.

It should be understood, however, that in some embodiments of theinvention lower edge and mixed dislocation densities than thosedescribed above may be achieved. It also should be understood that theabove-referenced edge dislocation densities may be present throughoutthe III-nitride material region(s) and, at least, in the same portionsof the III-nitride material region(s) having reduced screw dislocationdensities.

It is believed that the low screw dislocation densities are achieved, atleast in part, by the presence of the strain-absorbing layer. However,it should be understood that in certain embodiments of the inventionthat do not include the strain-absorbing layer, the III-nitride materialregions may also have the low screw dislocation densities. Other factorsinclude the composition, thickness and crystal structure of theintermediate layer, transition layer and III-nitride material region;the composition, thickness, and crystal structure of the substrate; and,process conditions described further below, amongst other factors.

In particular, the composition, thickness and crystal structure of thestrain-absorbing layer may contribute to reducing the formation of screwdislocations in the overlying III-nitride material layer(s)/region. Thestrain-absorbing layer may also reduce the formation of misfitdislocations in the layer formed on in the gallium nitride materialregion as described in commonly-owned, co-pending U.S. patentapplication serial number not yet assigned, filed Jun. 28, 2004,entitled “Gallium Nitride Materials and Methods Associated with theSame” which is incorporated herein by reference.

Strain-absorbing layer 12 helps absorb strain that arises due to latticedifferences between the crystal structure of the substrate and thecrystal structure of layer/region formed on the strain-absorbing layer(e.g., intermediate layer 15 in FIG. 1; transition layer 16 in FIG. 2;and gallium nitride material region in FIG. 3). In the absence of thestrain-absorbing layer, this strain is typically accommodated by thegeneration of misfit dislocations in the intermediate layer (or, otheroverlying layer) at the interface with the substrate. Thus, by providingan alternative mechanism for accommodating stress, the presence of thestrain-absorbing layer may reduce the generation of misfit dislocationsat the interface with the substrate. It is believed that the reductionin misfit dislocations can also lead to a reduction in screwdislocations in the overlying layers (e.g., intermediate layer 15,transition layer 16 and gallium nitride material region 18).

Furthermore, the strain-absorbing layer can help absorb strain thatarises due to differences in the thermal expansion rate of the substrateas compared to the thermal expansion rate of the intermediate layerand/or the gallium nitride material region. Such differences can lead toformation of misfit dislocations at the intermediate layer/substrateinterface, or cracking in either the intermediate layer and/or galliumnitride material region. As described further below, transition layer 16also helps absorb this thermally-induced strain.

Suitable strain-absorbing layers have been described in commonly-owned,co-pending U.S. patent application serial number not yet assigned, filedJun. 28, 2004, entitled “Gallium Nitride Materials and MethodsAssociated with the Same” which is incorporated by reference above. Incertain preferred embodiments, strain-absorbing layer 12 is formed of asilicon nitride-based material. Silicon nitride-based materials includeany silicon nitride-based compound (e.g., Si_(x)N_(y), such as SiN andSi₃N₄, SiCN, amongst others) including non-stoichiometric siliconnitride-based compounds. In some embodiments, a SiN strain-absorbinglayer may be preferred. Silicon nitride material-based strain-absorbinglayers may be particularly preferred when formed directly on a siliconsubstrate, as described further below.

It should also be understood that it is possible for thestrain-absorbing layer to be formed of other types of materialsaccording to other embodiments of the invention. Though all of theadvantages associated with silicon nitride-based materials may not beachieved in these embodiments.

In some embodiments, it is preferable for the strain-absorbing layer tohave an amorphous (i.e., non-crystalline) crystal structure. Amorphousstrain-absorbing layers are particularly effective in accommodatingstrain and, thus, reducing the generation of misfit dislocations.

However, it should be understood that in certain embodiments of theinvention, the strain-absorbing layer may have a single crystal orpoly-crystalline structure. In these cases, however, all of theadvantages associated with an amorphous strain-absorbing layer may notbe realized.

In some embodiments, it is preferred for the strain-absorbing layer tobe very thin, particularly when formed of amorphous and/or siliconnitride-based materials. It has been discovered that very thinstrain-absorbing layers (e.g., silicon nitride-based strain-absorbinglayers) may enable formation of intermediate layer(s) having anepitaxial relationship with the substrate, while also being effective inreducing the number of misfit dislocations (which may lead to thereduced screw dislocations in the III-nitride material region(s)). Incertain cases (e.g., when the strain-absorbing layer is amorphous), itis desirable for the strain-absorbing layer to have a thickness that islarge enough to accommodate sufficient strain associated with latticeand thermal expansion differences between the substrate and intermediatelayer 15 to reduce generation of misfit dislocations. In these cases, itmay also be desirable for the strain-absorbing layer to be thin enoughso that the intermediate layer has an epitaxial relationship with thesubstrate. This can be advantageous for formation of a high quality,single crystal III-nitride material region (e.g., gallium nitridematerial region). If the strain-absorbing layer is too thick, then theintermediate layer is not formed with epitaxial relationship with thesubstrate.

In some embodiments, the strain-absorbing layer has a thickness of lessthan about 100 Angstroms which, in these embodiments, can allow theepitaxial relationship between the substrate and the intermediate layer.In some embodiments, it may be preferable for the strain-absorbing layerto have a thickness of less than about 50 Angstroms to allow for theepitaxial relationship.

The strain-absorbing layer may have a thickness of greater than about 10Angstroms which, in these embodiments, is sufficient to accommodatestrain (e.g., resulting from lattice and thermal expansion differences)and can facilitate forming a strain-absorbing layer that covers theentire substrate, as described further below. In other embodiments, thestrain-absorbing layer may have a thickness of greater than about 20Angstroms to sufficiently accommodate strain.

Suitable thickness ranges for the strain-absorbing layer include all ofthose defined by the ranges described above (e.g., greater than about 10Angstroms and less than about 100 Angstroms, greater than about 10Angstroms and less than about 50 Angstroms, and the like). Also, thestrain-absorbing layer thickness may be between about 20 Angstroms andabout 70 Angstroms.

It should be understood that suitable thicknesses of thestrain-absorbing layer may depend on a number of factors including thecomposition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the intermediate layer;as well as the composition, thickness, and crystal structure of thesubstrate, amongst other factors. Suitable thicknesses may be determinedby measuring the effect of thickness on misfit dislocation density andother factors (e.g., the ability to deposit an intermediate layer havingan epitaxial relationship with the substrate, etc.). It is also possiblefor the strain-absorbing layer to have a thickness outside the aboveranges.

In some cases, the thickness of the strain-absorbing layer is relativelyuniform across the entire layer. For example, in these cases, thestrain-absorbing layer may have a thickness uniformity variation of lessthan 25 percent, or less than 10 percent, across the entirestrain-absorbing layer.

As described further below, in some embodiments, the strain-absorbinglayer may be formed by nitridating a top surface region of a siliconsubstrate. That is, the surface region of the substrate may be convertedfrom silicon to a silicon nitride-based material to form thestrain-absorbing layer. It should be understood that, as used herein,such strain-absorbing layers may be referred to as being “formed on thesubstrate”, “formed over the substrate”, “formed directly on thesubstrate” and as “covering the substrate”. Such phrases also refer tostrain-absorbing layers that are formed by depositing a separate layer(e.g., using a separate nitrogen source and silicon source) on the topsurface of the substrate and are not formed by converting a surfaceregion of the substrate.

In the illustrative embodiment, the strain-absorbing layer coverssubstantially the entire top surface of the substrate. This arrangementmay be preferable to minimize the number of misfit dislocations in theintermediate layer and, thus, the number of screw dislocations in theIII-nitride material region(s). In other embodiments, thestrain-absorbing layer may cover a majority of the top surface of thesubstrate (e.g., greater than 50 percent or greater than 75 percent ofthe top surface area).

Also, in the illustrative embodiment, strain-absorbing layer 12 isformed across the entire area between the substrate and the intermediatelayer. That is, the strain-absorbing layer separates the substrate andthe intermediate layer at all points with the strain-absorbing layerbeing directly on the substrate and the intermediate layer beingdirectly on the strain-absorbing layer. This arrangement may bepreferable to minimize the number of misfit dislocations in theintermediate layer. In other embodiments, the strain-absorbing layer maybe formed across a majority of the area (e.g., greater than 50 percent,or greater than 75 percent) between the substrate and the intermediatelayer. If the strain-absorbing layer is not present across the entire(or, at least, the majority of the) area between the substrate and theintermediate layer, the above-noted advantages associated with thestrain-absorbing layer may not be realized.

The extent that the strain-absorbing layer covers the substrate (and thearea between the intermediate layer and the substrate) in the presentinvention may be distinguished from certain prior art techniques inwhich a discontinuous silicon nitride layer is formed (in some cases,inadvertently) between a silicon substrate and an overlying layer.

It should be understood that, in other embodiments, the strain-absorbinglayer may be positioned in other locations such as between two differentlayers (i.e., not formed directly on the substrate). In theseembodiments, the strain-absorbing layer may reduce the formation ofmisfit dislocations and reduce the propagation of screw dislocations tothe layer(s)/region(s) that overlie the strain-absorbing layer. Thestrain-absorbing layer may cover the layer on which it is formed to asimilar extent as described above in connection with thestrain-absorbing layer covering the substrate.

As noted above, the presence of the strain-absorbing layeradvantageously results in very low misfit dislocation densities withinthe intermediate layer (e.g., at, or very near, an interface between thestrain-absorbing layer and the intermediate layer). In some embodimentsof the invention, the misfit dislocation density in the overlying layeris less than about 10¹⁰/cm²; and, in other embodiments, less than about10⁸/cm². Even lower misfit dislocation densities in the overlying layermay be achieved, for example, less than about 10⁵/cm². In some cases,the presence of misfit dislocations may not be readily detectable whichgenerally means that the misfit dislocation density is less than about10²/cm². The specific misfit dislocation density depends, in part, onthe particular structure including factors such as the thickness,composition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the overlying layer; aswell as the composition, thickness, and crystal structure of thesubstrate, amongst other factors.

It may be preferred for structure 10 to include an intermediate layer 15formed of a nitride-based material. Suitable nitride-based materialsinclude, but are not limited to, aluminum nitride-based materials (e.g.,aluminum nitride, aluminum nitride alloys) and gallium nitridematerials. In some cases, the intermediate layer has a constantcomposition. In other cases, as described further below, theintermediate layer may be compositionally-graded. Suitablecompositionally-graded layers are described further below and have beendescribed in commonly-owned U.S. Pat. No. 6,649,287, entitled “GalliumNitride Materials and Methods” filed on Dec. 14, 2000, which isincorporated herein by reference.

It may be preferable for the intermediate layer to have a single crystalstructure. As noted above, in some embodiments, the thickness of thestrain-absorbing layer is controlled so that the intermediate layer hasan epitaxial relationship with the substrate. It may be advantageous forthe intermediate layer to have a single crystal structure because itfacilitates formation of a single crystal, high quality gallium nitridematerial region having low screw dislocation densities. In someembodiments, the intermediate layer has a different crystal structurethan the substrate.

It should also be understood that the intermediate layer may not have asingle crystal structure and may be amorphous or polycrystalline, thoughall of the advantages associated with the single crystal intermediatelayers may not be achieved.

The intermediate layer may have any suitable thickness. For example, theintermediate layer may be between about 10 nanometers and 5 microns,though other thicknesses are also possible.

It should be understood that certain embodiments of the invention maynot include an intermediate layer 15 (e.g., as shown in FIG. 2).

In the illustrative embodiment, transition layer 16 is formed directlyon the intermediate layer. In certain embodiments, such as when theintermediate layer has a constant composition, it may be preferred forthe transition layer to be formed of a compositionally-graded material(e.g., a compositionally-graded nitride-based material). Suitablecompositionally-graded layers have been described in commonly-owned U.S.Pat. No. 6,649,287 which is incorporated by reference above.

Compositionally-graded transition layers have a composition that isvaried across at least a portion of the layer. Compositionally-gradedtransition layers are particularly effective in reducing crack formationin gallium nitride material regions formed on the transition layer bylowering thermal stresses that result from differences in thermalexpansion rates between the gallium nitride material and the substrate(e.g., silicon). Compositionally-graded transition layers may alsocontribute to reducing generation of screw dislocations in theIII-nitride material layer(s)/region(s) (e.g., gallium nitride materialregion). In some cases, the compositionally-graded transition layers mayalso contribute to reducing mixed and edge dislocation densities.

According to one set of embodiments, the transition layer iscompositionally-graded and formed of an alloy of gallium nitride such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. Inthese embodiments, the concentration of at least one of the elements(e.g., Ga, Al, In) of the alloy is varied across at least a portion ofthe thickness of the transition layer. When transition layer 16 has anAl_(x)In_(y)Ga_((1-x-y))N composition, x and/or y may be varied. Whenthe transition layer has a Al_(x)Ga_((1-x))N composition, x may bevaried. When the transition layer has a In_(y)Ga_((1-y))N composition, ymay be varied.

In certain preferred embodiments, it is desirable for the transitionlayer to have a low gallium concentration at a back surface which isgraded to a high gallium concentration at a front surface. It has beenfound that such transition layers are particularly effective inrelieving internal stresses within gallium nitride material region 18.For example, the transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is decreased from the back surface to thefront surface of the transition layer (e.g., x is decreased from a valueof 1 at the back surface of the transition layer to a value of 0 at thefront surface of the transition layer).

In some preferred embodiments, structure 10 includes an aluminum nitrideintermediate layer 15 and a compositionally-graded transition layer 16.In some preferred embodiments, the compositionally-graded transitionlayer has a composition of Al_(x)Ga_((1-x))N, where x is graded from avalue of 1 at the back surface of the transition layer to a value of 0at the front surface of the transition layer. The composition, forexample, can be graded discontinuously (e.g., step-wise) orcontinuously. One discontinuous grade may include steps of AN,Al_(0.6)Ga_(0.4)N and Al_(0.3)Ga_(0.7)N proceeding in a direction towardthe gallium nitride material region.

It should be understood that, in other cases, transition layer 16 mayhave a constant composition and may not be compositionally-graded. Insome cases (e.g., when the substrate is not a silicon substrate), thetransition layer may have a constant composition. Suitable compositionsinclude, but are not limited to, aluminum nitride-based materials (e.g.,aluminum nitride, aluminum nitride alloys) and gallium nitridematerials. In these constant composition embodiments, the transitionlayer may be similar to the intermediate layer in other illustrativeembodiments.

The strain absorbing layer, intermediate layer and transition layer arenot typically (though may be) part of the active region of devicesformed from structures of the invention. As described above, theselayers may be formed to facilitate deposition of gallium nitridematerial region 18. However, in some cases, the intermediate layerand/or transition layer may have other functions including functioningas a heat spreading layer that helps remove heat from active regions ofthe semiconductor structure during operation of a device. For example,such transition layers that function as heat spreading layers have beendescribed in commonly-owned, co-pending U.S. patent application Ser. No.09/792,409 entitled “Gallium Nitride Materials IncludingThermally-Conductive Regions,” filed Feb. 23, 2001, which isincorporated herein by reference. Active regions of devices formed fromthe structure of the invention may be formed in gallium nitride materialregion 18. Gallium nitride material region 18 comprises at least onegallium nitride material layer. In some cases, gallium nitride materialregion 18 includes only one gallium nitride material layer. In othercases, gallium nitride material region 18 includes more than one galliumnitride material layer. For example, the gallium nitride material regionmay include multiple layers (e.g., 18 a, 18 b, 18 c) as shown in FIG. 5.In embodiments having more than one gallium nitride material layer, eachlayer may have the reduced screw dislocation densities noted above.

Suitable gallium nitride material layer arrangements have beendescribed, for example, in commonly-owned, co-pending U.S. patentapplication Ser. No. 10/740,376 entitled “Gallium Nitride MaterialDevices Including an Electrode-Defining Layer and Methods of Forming theSame,” filed Dec. 17, 2003, which is incorporated herein by reference.

Preferably, gallium nitride material region 18 also has a low cracklevel in addition to the low screw dislocation densities noted above. Asdescribed above, the transition layer (particularly whencompositionally-graded) and/or intermediate layer may reduce crackformation. Gallium nitride materials having low crack levels have beendescribed in U.S. Pat. No. 6,649,287 incorporated by reference above. Insome cases, the gallium nitride material region has a crack level ofless than 0.005 μm/μm². In some cases, the gallium nitride materialregion has a very low crack level of less than 0.001 μm/μm². In certaincases, it may be preferable for the gallium nitride material region tobe substantially crack-free as defined by a crack level of less than0.0001 μm/μm².

In certain cases, gallium nitride material region 18 includes a layer(or layers) which have a single crystal (i.e., monocrystalline)structure. In some cases, the gallium nitride material region includesone or more layers having a Wurtzite (hexagonal) structure.

The thickness of gallium nitride material region 18 and the number ofdifferent layers are dictated, at least in part, on the application inwhich the structure is used. At a minimum, the thickness of the galliumnitride material region is sufficient to permit formation of the desiredstructure or device. The gallium nitride material region generally has athickness of greater than 0.1 micron, though not always. In other cases,gallium nitride material region 18 has a thickness of greater than 0.5micron, greater than 2.0 microns, or even greater than 5.0 microns. Insome cases, gallium nitride material region has a thickness of less than10.0 microns, or less than 5.0 microns. It may be advantageous for thegallium nitride material region to have thicknesses less than thesevalues to limit processing times and for other processing reasons.

In some embodiments, the III-nitride material layer(s)/region(s) (e.g.,gallium nitride material regions) may comprise limited amounts, or besubstantially free, of additives (other than dopants). For example, thegallium nitride material may include less than about 0.01 ppm of alkalimetal elements, or may be substantially free of alkali metal elements.This is an advantage over certain prior art processes, which may haveused alkali metals in III-nitride material production, due to theelimination of impurities.

As shown, the layers/regions of the device (e.g., strain-absorbinglayer, intermediate layer, transition layer, gallium nitride materialregion) may be substantially planar in the final device or structure. Asdescribed further below, substantially planar layers/regions may begrown in vertical (e.g., non-lateral) growth processes. Certain lateralgrowth processes may lead to structures having non-planar layers/regions(e.g., as shown in U.S. Pat. No. 6,265,289). Planar layers/regions (andsubstrates, as noted below) can be advantageous in some embodiments, forexample, in forming gallium nitride material regions havingsubstantially constant screw dislocation densities and in simplifyingprocessing, amongst other advantages. Though it should be understoodthat, in some embodiments of the invention, lateral growth processes maybe used (in some cases, combined with vertical growth processes) asdescribed further below.

The layers/regions may form a substantially planar interface withrespective underlying layers/regions (including substrate). For example,the gallium nitride material region may form a substantially planarinterface with transition layer 16; and, strain-absorbing layer 12 orthe transition layer 16 may form a substantially planar interface withthe substrate. This also may be indicative of a vertical growth process,in contrast with structures formed in lateral growth processes (e.g., asshown in U.S. Pat. No. 6,265,289). Forming a substantially planarinterface between the gallium nitride material region and the underlyinglayer (e.g., transition layer 16) may lead to several advantagesincluding promoting formation of substantially constant screwdislocation densities in the gallium nitride material region andsimplifying processing, amongst other advantages.

It should be understood that the structures of the present invention arenot limited to substantially planar layers/regions and/or interfaces. Insome embodiments, the structures include non-planar layers/regionsand/or interfaces.

In some embodiments, the layers/regions may form a continuous interfacewith respective underlying layers/regions (including substrate). Thatis, the layers/regions of the structure (e.g., strain-absorbing layer,intermediate layer, transition layer, gallium nitride material region)may continuously extend across respective underlying layers/regions. Forexample, the gallium nitride material region may form a continuousinterface with transition layer 16; and, strain-absorbing layer 12 orthe transition layer 16 may form a continuous interface with thesubstrate. The layers/regions may also continuously cover the topsurface of the respective underlying layers/regions.

As described above, in certain preferred embodiments, substrate 14 is asilicon substrate. As used herein, a silicon substrate refers to anysubstrate that includes a silicon surface. Examples of suitable siliconsubstrates include substrates that are composed entirely of silicon(e.g., bulk silicon wafers), silicon-on-insulator (SOI) substrates,silicon-on-sapphire substrate (SOS), and SIMOX substrates, amongstothers. Suitable silicon substrates also include substrates that have asilicon wafer bonded to another material such as diamond, AlN, or otherpolycrystalline materials. Silicon substrates having differentcrystallographic orientations may be used, though single crystal siliconsubstrates are preferred. In some cases, silicon (111) substrates arepreferred. In other cases, silicon (100) substrates are preferred.

It should be understood that other types of substrates may also be usedincluding sapphire, silicon carbide, indium phosphide, silicongermanium, gallium arsenide, gallium nitride, aluminum nitride and otherIII-V compound substrates. However, in embodiments that do not usesilicon substrates, all of the advantages associated with siliconsubstrates may not be achieved. In some embodiments, it may bepreferable to use non-nitride material-based substrates such as silicon,sapphire, silicon carbide, indium phosphide, silicon germanium andgallium arsenide.

In some cases (e.g., FIG. 4), as described further below, the substrate(and other layers) may be removed to form a free-standing III-nitridematerial region having low screw dislocation densities.

Substrate 14 may have any suitable dimensions. Suitable diameters mayinclude, but are not limited to, about 2 inches (50 mm), 4 inches (100mm), 6 inches (150 mm), and 8 inches (200 mm). Advantageously, thestrain-absorbing layer may be used to form a high quality galliumnitride material region on substrates (e.g., silicon substrates) over avariety of thicknesses. In some cases, it may be preferable for thesubstrate to be relatively thick, such as greater than about 125 micron(e.g., between about 125 micron and about 800 micron, or between about400 micron and 800 micron). Relatively thick substrates may be easy toobtain, process and can resist bending which can occur, in some cases,in thinner substrates. In other embodiments, thinner substrates (e.g.,less than 125 microns) are used, though these embodiments may not havethe advantages associated with thicker substrates, but can have otheradvantages including facilitating processing and/or reducing the numberof processing steps. In some processes, the substrate initially isrelatively thick (e.g., between about 200 microns and 800 microns) andthen is thinned during a later processing step (e.g., to less than 150microns).

In some preferred embodiments, the substrate is substantially planar inthe final device or structure. Substantially planar substrates may bedistinguished from substrates that are textured and/or have trenchesformed therein (e.g., as in U.S. Pat. No. 6,265,289). Substantiallyplanar substrates may facilitate formation of substantially planarlayers/regions thereupon.

FIG. 2 illustrates a semiconductor structure 20 according to anotherembodiment of the invention. Semiconductor structure 20 is similar tothat shown in FIG. 1 except the structure does not include intermediatelayer 15.

FIG. 3 illustrates a semiconductor structure 22 according to anotherembodiment of the invention. Semiconductor structure 20 is similar tothat shown in FIG. 1 except the structure does not include intermediatelayer 15 or transition layer 16.

FIG. 4 illustrates a structure 40 that comprises a free-standing galliumnitride material region 18. The free-standing gallium nitride materialregion may be formed by removing the substrate and layers (e.g., 12, 15,16) underlying the gallium nitride material region. For example, thesubstrate and layers may be removed by etching processes. In thisembodiment, the gallium nitride material region may have any of thecharacteristics described above including the low screw dislocationdensities. In particular, it may be advantageous for the free-standinggallium nitride material region to have a substantially constant lowscrew dislocation density. Structure 40 may be further processed, forexample, to form semiconductor devices.

Though FIG. 4 shows a gallium nitride material region 18, it should beunderstood that any of the other III-nitride materials described hereinmay also be converted to a free-standing structure.

The semiconductor structures illustrated in FIGS. 1-4 may form the basisof a variety of semiconductor devices. Suitable devices include, but arenot limited to, transistors (e.g., FETs), as well as light-emittingdevices including LEDs and laser diodes. The devices have active regionsthat are typically, at least in part, formed within the gallium nitridematerial region. Also, the devices include a variety of other functionallayers and/or features (e.g., electrodes).

For example, FIG. 5 illustrates a FET device 30 according to oneembodiment of the invention. Device 30 includes a source electrode 34, adrain electrode 36 and a gate electrode 38 formed on gallium nitridematerial region 18 (which includes a first layer 18 b and a second layer18 a). The device also includes an electrode defining layer 40 which, asshown, is a passivating layer that protects and passivates the surfaceof the gallium nitride material region. A via 42 is formed within theelectrode defining layer in which the gate electrode is, in part,formed. Strain-absorbing layer 12 is formed directly on the substrateand intermediate layer 15 is formed directly on the strain-absorbinglayer. In some embodiments, the intermediate layer iscompositionally-graded. In some embodiments, the intermediate layer mayhave a constant composition (e.g., aluminum nitride or an aluminumnitride alloy) and a compositionally-graded transition layer is formedon the strain-absorbing layer. As shown, the gallium nitride materialregion includes multiple gallium nitride material layers (18 a, 18 b, 18c) with each layer having a low screw dislocation density.

In certain embodiments, it may be preferable for the gallium nitridematerial of layer 18 b to have an aluminum concentration that is greaterthan the aluminum concentration of the gallium nitride material of layer18 a. For example, the value of x in the gallium nitride material oflayer 18 b (with reference to any of the gallium nitride materialsdescribed above) may have a value that is between 0.05 and 1.0 greaterthan the value of x in the gallium nitride material of layer 18 a; or,between 0.05 and 0.5 greater than the value of x in the gallium nitridematerial of layer 18 a. For example, layer 18 b may be formed ofAl_(0.26)Ga_(0.74)N, while layer 18 a is formed of GaN. This differencein aluminum concentration may lead to formation of a highly conductiveregion at the interface of the layers 18 b, 18 a (i.e., a 2-D electrongas region). In the illustrative embodiment, layer 18 c may be formed ofGaN.

It should be understood that other structures and devices may be withinthe scope of the present invention including structures and devices thatare not specifically described herein. Other structures may includeother layers and/or features, amongst other differences.

Semiconductor structure 10 may be manufactured using known semiconductorprocessing techniques. In embodiments in which the strain-absorbinglayer is a silicon nitride-based material (e.g., amorphous SiN), thestrain-absorbing layer may be formed by nitridating a top surface of thesilicon substrate as noted above. In a nitridation process, nitrogenreacts with a top surface region of the silicon substrate to form asilicon nitride-based layer. The top surface may be nitridated byexposing the silicon substrate to a gaseous source of nitrogen atelevated temperatures. For example, ammonia may be introduced into areaction chamber in which a silicon substrate is positioned. Thetemperature in the reaction chamber may be between about 1000° C. andabout 1100° C. and the pressure may be between about 20 torr and about40 torr (in some cases, about 30 torr). The reaction between nitrogenand the silicon substrate is allowed to proceed for a reaction timeselected to produce a layer having a desired thickness.

It should be understood that other processes may be used to form siliconnitride-based strain-absorbing layers including processes (e.g., CVDprocesses) that use separate nitrogen and silicon sources. Also, whenthe strain-absorbing layer is formed of another type of material(non-silicon nitride-based material), other deposition processes knownin the art are used.

In some embodiments, the strain-absorbing layer may be formed in-situwith the intermediate layer (and, in some cases, subsequent layers) ofthe structure. That is, the strain-absorbing layer may be formed duringthe same deposition step as the intermediate layer (and, in some cases,subsequent layers). In processes that grow a silicon nitride-basedmaterial strain-absorbing layer by introducing a nitrogen source (e.g.,ammonia) into a reaction chamber as described above, a second source gasmay be introduced into the chamber after a selected time delay after thenitrogen source. The second source reacts with the nitrogen source toform the intermediate layer, thus, ending growth of the strain-absorbinglayer. For example, when the intermediate layer is formed of aluminumnitride, an aluminum source (e.g., trimethylaluminum) is introduced intothe chamber at a selected time after the nitrogen source (e.g.,ammonia). The time delay is selected so that the strain-absorbing layergrows to a desired thickness. The reaction between the second source(e.g., aluminum source) and the nitrogen source is allowed to proceedfor a sufficient time to produce the intermediate layer. When theintermediate layer has a single crystal structure, the reactionconditions are selected appropriately. For example, the reactiontemperature may be greater than 700° C., such as between about 1000° C.and about 1100° C. In some cases, lower growth temperatures may be usedincluding temperatures between about 500° C. and about 600° C.

It should also be understood that the strain-absorbing layer may beformed in a separate process than the intermediate layer and subsequentlayers. For example, the strain-absorbing layer may be formed on thesubstrate in a first process. Then, at a later time, the intermediatelayers may be formed on the strain-absorbing layer in a second process.

In the processes described above, the intermediate layer is grown in avertical growth process. That is, the intermediate layer is grown in avertical direction (i.e., along y-axis in FIG. 1) with respect to thestrain-absorbing layer. The ability to vertically grow thestrain-absorbing layer having low misfit dislocation densities (whichcan contribute to the low screw dislocation densities) may beadvantageous as compared to lateral growth processes which may be morecomplicated.

Transition layer 16 and gallium nitride material region 18 may also begrown in the same deposition step as the intermediate layer and thestrain-absorbing layer. In such processes, suitable sources areintroduced into the reaction chamber at appropriate times.

According to one preferred method, the transition layer and galliumnitride material region (and intermediate layers, if present) are grownusing a metalorganic chemical vapor deposition (MOCVD) process. Itshould be understood that other suitable techniques known in the art mayalso be utilized to deposit the transition layer and the gallium nitridematerial region including molecular beam epitaxy (MBE), hydride vaporphase epitaxy (HVPE), and the like.

Generally, the MOCVD process involves introducing different source gasesinto an environment (e.g., a process system) and providing conditionswhich promote a reaction between the gases to form a layer. The reactionproceeds until a layer of desired thickness is achieved. The compositionof the layer may be controlled, as described further below, by severalfactors including gas composition, gas concentration, and the reactionconditions (e.g., temperature and pressure).

Examples of suitable source gases for MOCVD growth of the transitionlayer and gallium nitride material region. include trimethylaluminum(TMA) or triethylaluminum (TEA) as sources of aluminum; trimethylindium(TMI) or triethylindium (TEI) as sources of indium; trimethylgallium(TMG) or trimethylgallium (TEG) as sources of gallium; and ammonia (NH₃)as a source of nitrogen. The particular source gas used depends upon thedesired composition of the layers. For example, an aluminum source(e.g., TMA or TEA), a gallium source (TMG or TEG), and a nitrogen sourceare used to deposit films having an Al_(x)Ga_(1-x)N composition.

The flow rates of the source gases, the ratios of the source gases, andthe absolute concentrations of the source gases may be controlled toprovide transition layers and gallium nitride material regions havingthe desired composition. For the growth of Al_(x)Ga_(1-x)N layers,typical TMA flow rates are between about 5 μmol/min and about 50μmol/min with a flow rate of about 20 μmol/min being preferred in somecases; typical TMG flow rates are between about 5 μmol/min and 250μmol/min, with a flow rate of 115 μmol/min being preferred in somecases; and the flow rate of ammonia is typically between about 3 slpm toabout 10 slpm. The reaction temperatures are generally between about900° C. and about 1200° C. and the process pressures are between about 1Torr and about 760 Torr. It is to be understood that the processconditions, and in particular the flow rate, are highly dependent on theprocess system configuration. Typically, smaller throughput systemsrequire less flow than larger throughput systems.

When forming a compositionally-graded transition layer, processparameters may be suitably adjusted to control the compositionalgrading. The composition may be graded by changing the processconditions to favor the growth of particular compositions. For example,to increase incorporation of gallium in the transition layer therebyincreasing the gallium concentration, the flow rate and/or theconcentration of the gallium source (e.g., TMG or TEG) may be increased.Similarly, to increase incorporation of aluminum into the transitionlayer thereby increasing the aluminum concentration, the flow rateand/or the concentration of the aluminum source (e.g., TMA or TEA) maybe increased. The manner in which the flow rate and/or the concentrationof the source is increased (or decreased) controls the manner in whichthe composition is graded. In other embodiments, the temperature and/orpressure is adjusted to favor the growth of a particular compound.Growth temperatures and pressures favoring the incorporation of galliuminto the transition layer differ from the growth temperatures andpressures favoring the incorporation of aluminum into the transitionlayer. Thus, the composition may be graded by suitably adjustingtemperature and pressure.

When depositing a transition layer or a gallium nitride material region(or an intermediate layer) having a constant composition, however, theprocess parameters are maintained constant so as to provide a layerhaving a constant composition. When gallium nitride material regionsinclude more than one gallium nitride material layer having differentrespective compositions, the process parameters may be changed at theappropriate time to change the composition of the layer being formed.

It should be understood that all of the layers/regions on the substratemay be grown in the same process (i.e., the strain-absorbing layer,intermediate layer, transition layer and the gallium nitride materialregion). Or, respective layers/regions may be grown separately.

The processes described involve growing the layers/regions (i.e., thestrain-absorbing layer, intermediate layer, transition layer and thegallium nitride material region) in vertical growth processes. That is,these layers/regions are grown in a vertical direction with respect tounderlying layers/regions (including substrate). The ability tovertically grow the layers/regions including the gallium nitridematerial region having low screw dislocation densities may beadvantageous as compared to lateral growth processes which may be morecomplicated.

However, in other embodiments of the invention (not shown), it ispossible to grow, at least a portion of, gallium nitride material region18 using a lateral epitaxial overgrowth (LEO) technique that involvesgrowing an underlying gallium nitride layer through mask openings andthen laterally over the mask to form the gallium nitride materialregion, for example, as described in U.S. Pat. No. 6,051,849.

In other embodiments of the invention (not shown), it is possible togrow the gallium nitride material region 18 using a pendeoepitaxialtechnique that involves growing sidewalls of gallium nitride materialposts into trenches until growth from adjacent sidewalls coalesces toform a gallium nitride material region, for example, as described inU.S. Pat. No. 6,265,289. In these embodiments, for example,strain-absorbing layer 12 (and, in some cases, layers 15, 16) may bedeposited on the sidewalls prior to lateral growth which may reducescrew dislocation densities in laterally-grown III-nitride materiallayers/regions (e.g., gallium nitride material regions).

Commonly-owned, co-pending U.S. patent application Ser. No. 10/740,376entitled “Gallium Nitride Material Devices Including anElectrode-Defining Layer and Methods of Forming the Same,” filed Dec.17, 2003, which is incorporated herein by reference, further describestechniques used to grow other layers and features shown in theembodiment of FIG. 5.

It should also be understood that other processes may be used to formstructures and devices of the present invention as known to those ofordinary skill in the art.

The following example is meant to be illustrative and is not limiting.

EXAMPLE 1

This example illustrates the formation of a structure includingIII-nitride material layers/regions having low screw dislocationdensities according to one embodiment of the present invention.

A 100 mm silicon substrate was placed in a reaction chamber. Ammonia gaswas introduced into the chamber as a nitrogen source. The temperaturewas maintained at 1030° C. and the pressure at about 30 torr. Astrain-absorbing layer of amorphous silicon nitride (SiN) was formed.

About 6 seconds after the introduction of ammonia, TMA was introducedinto the chamber as an aluminum source. The temperature and pressurewere respectively maintained at 1030° C. and about 30 torr. Growthproceeded for about 60 minutes. An aluminum nitride (AlN) was formed.

After about 60 minutes, TMG was introduced into the chamber. Growthproceeded for about 40 minutes. An aluminum gallium nitride layer(Al_(0.6)Ga_(0.4)N) was formed.

After about 40 minutes, the concentration of TMG being introduced intothe chamber was increased and the concentration of TMA being introducedinto the chamber was decreased. Growth proceeded for 20 minutes. Analuminum gallium nitride layer (Al_(0.3)Ga_(0.7)N) was formed.

After about 20 minutes, the supply of TMA was turned off and only TMGwas introduced into the chamber. A gallium nitride (GaN) layer wasformed. Growth proceeded for 30 minutes

The supply of TMA was turned back on and TMA and TMG were introducedinto the chamber to grow a thin aluminum gallium nitride layer.

FIG. 6 is a copy of a STEM (scanning transmission electron microscope)image that illustrates the presence of a silicon nitridestrain-absorbing layer between the aluminum nitride layer and thesilicon substrate.

A g-vector analysis was used to assess screw dislocation density ofoverlying III-nitride material layers/regions using conventional TEMtechniques. FIGS. 7A-8B are copies of TEM images taken of verticalcross-sections of the same structure. For reference, the “*” indicatesidentical spatial positions on the images.

FIG. 7A is an image with the g-vector (g=(00001)) perpendicular to thesubstrate surface. FIG. 7B is an image with the g-vector (g=((1-100))parallel to the substrate surface.

FIGS. 8A and 8B are respective copies of FIGS. 7A and 7B withdislocations present in the gallium nitride material region and a firstportion of the transition layer (Al_(0.3)Ga_(0.7)N) highlighted bydashed lines. It should be noted that the second portion of AlGaNtransition layer and the AN layer indicate similar dislocationdensities, but dashed lines were not individually noted for the sake ofclarity.

As shown, all of the dislocations present in FIGS. 7A and 8A are alsofound in FIGS. 7B and 8B. This one-to-one correspondence indicates thatall of these dislocations are edge dislocations or mixed dislocations(which include an edge component). Thus, no screw dislocations wereobserved in the TEM images.

Also, there are a number of dislocations present in FIGS. 7B and 8Bwhich are not present in FIGS. 7A and 8A. This is also indicative ofthese dislocations being edge dislocations or mixed dislocations.

This example establishes formation of a III-nitride materiallayers/regions having a very low screw dislocation density. No screwdislocations were detected in the TEM images.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A semiconductor structure comprising: a III-nitride material regionhaving a screw dislocation density of less than about 10⁸/cm² throughoutthe entire volume of the III-nitride material region and having an edgedislocation density of greater than about 10⁸/cm².
 2. The structure ofclaim 1, wherein the III-nitride material region is a gallium nitridematerial region.
 3. The structure of claim 1, wherein the III-nitridematerial region is free-standing.
 4. The structure of claim 1, whereinthe III-nitride material region is substantially planar.
 5. Thestructure of claim 1, wherein the III-nitride material region isvertically-grown.
 6. The structure of claim 1, wherein the III-nitridematerial region extends continuously across an underlying region.